linux-kernel - Re: [PATCH] CIFS: Decrease reconnection delay when switching nics

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Date:	Thu, 28 Feb 2013 20:54:12 -0600
From:	Steve French <smfrench@...il.com>
To:	Jeff Layton <jlayton@...ba.org>
Cc:	Björn JACKE <bj@...net.de>,
	Dave Chiluk <dave.chiluk@...onical.com>,
	Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org,
	"Stefan (metze) Metzmacher" <metze@...ba.org>,
	Dave Chiluk <chiluk@...onical.com>
Subject: Re: [PATCH] CIFS: Decrease reconnection delay when switching nics

On Thu, Feb 28, 2013 at 6:11 PM, Jeff Layton <jlayton@...ba.org> wrote:
> On Thu, 28 Feb 2013 23:54:13 +0100
> Björn JACKE <bj@...Net.DE> wrote:
>
>> On 2013-02-28 at 07:26 -0800 Jeff Layton sent off:
>> > NTFS doesn't support sparse files, so the OS has to zero-fill up to the
>> > point where you're writing. That can take a looooong time on slow
>> > storage (minutes even).
>>
>> but you are talking about FAT here, right? NTFS does support sparse files if
>> the sparse bit has been explicitly been set on it. Bit even if the sparse bit
>> is not set filling a file with zeros by writing after a seek long beyond the
>> end of the file is very fast because NTFS supports that feature what Unix
>> filesystems like xfs call extents.
>>
>> If writing beyond the end of a file is really slow via cifs vfs in the test
>> case against a ntfs volume then I wonder if that operation is being really done
>> optimally over the wire. ntfs really isn't that bad with handling this kind of
>> files.
>>
>
> I'm not sure since I don't know the internals of NTFS. I had always
> assumed that it didn't really handle sparse files well (hence the
> "rabbit-pellet" thing that windows clients do).
>
> All I can say however is that writes long past the EOF can take a
> *really* long time to run. Typically we just issue a SMB_COM_WRITEX at
> the offset to which we want to put the data. Is there some other way we
> ought to be doing this?
>
> In any case, it doesn't really change the fact that there is no
> guaranteed time of response from CIFS servers. They can easily take a
> really long time to respond to certain requests. The best method we
> have to deal with that is to periodically "ping" the server with an
> echo to see if it's still there.

SMB2/SMB3 with better async support may make this easier - but Jeff is right.

-- 
Thanks,

Steve
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